The present invention relates to an a.c. variable speed drive system for driving a wound-rotor type induction motor at variable speed.
As a well known method of driving a wound-rotor type induction motor at variable speed, there is the method of regulating the secondary resistor of the wound-rotor type induction motor by using a resistor, and the method of regenerating the secondary power of the wound-rotor type induction motor at the commercial power supply by using a Scherbius apparatus or the like.
The method using a resistor is associated with the problems that power is consumed by the resistor so that the efficiency of the overall system lowers, and that maintenance labor is required for the resistor.
The method using the Scherbius apparatus is also associated with the problem that although the rating of the apparatus is small if the range of speed variation is narrow, if a wide range of speed variation is necessary, the rating of the Scherbius apparatus becomes too large and not economical, and moreover it becomes necessary to use a starting resistor such as a liquid type resistor.